1. Field of the Invention
The present invention relates to a curable resin composition, a water-soluble ink composition, an ink set, and an image-forming method.
2. Description of the Related Art
A curable resin composition, which crosslinks a binder and the like in association with a polymerizable compound having an ethylenic unsaturated group as a crosslinking agent when imparted with energy such as light or heat and is cured, has been widely used for industrial usages such as a coating material, paint, printing ink, an adhesive, an optical material, a laminating material for electronic products, an electrical insulating material, and a resist material. A cross-linking compound is cured when irradiated with an active energy ray such as an ultraviolet ray. For example, the cross-linking compound as a curing agent or a crosslinking agent is blended into ink or paint, and after an image is formed, the cross-linking compound is polymerized by being irradiated with an ultraviolet ray or the like so as to form a cured film, thereby improving the weather resistance or durability of the image.
Among the curable resin compositions, for ink compositions, recently, there has been a demand for reducing a volatile organic compound (VOC), which has been widely used in the related art as a solvent or the like, from the viewpoint of environmental protection or safety, and accordingly, a waterborne photocurable system capable of reducing the usage amount of the VOC is attracting attention. Even in the field of printing ink, there have been a variety of proposals regarding waterborne photocurable ink in which a water-soluble solvent such as water or an alcohol is used as a medium, and the curable resin compositions are also applied to ink jet recording methods.
In the ink jet recording method, since a printing apparatus is inexpensive, no plate is required during printing, and an image is directly formed on a recording medium by discharging ink only to required image sections, ink can be efficiently used, and there is an advantage of having inexpensive operational costs particularly in the case of small lot production. Furthermore, only a small amount of noise is generated, and recently, the method has been attracting attention as an excellent image-recording method.
In ink for photocurable ink jet recording, generally, an image is formed using an aqueous ink containing a polymerizable compound and a polymerization initiator, and the image is cured and fixed by radiating light. Light in the ultraviolet (UV) range is generally used as a light source. Since a majority of ink components is cured through light radiation, drying properties are superior to those of solvent-based ink, images do not easily bleed, and thus the ink can be printed on a variety of recording media, which makes the ink jet recording method excellent.
In the above-described waterborne ink for photocurable ink jet recording, a water-soluble photopolymerization initiator is used; however, among photopolymerization initiators that are commercially available at the present time, only a limited range of photopolymerization initiators such as α-hydroxyacetophenone-based initiators are being used, and the degree of solubility in water is also insufficient. As a result, there has been a proposal regarding the introduction of an ionic substituent such as carboxylate or sulfonate (refer to JP1994-228218A (JP-H6-228218A), JP2008-247939A, and GB2108487B), but the temporal stability of ink easily degrades (ink easily aggregates), and it cannot be said that this method has been satisfactory at all times. In addition, recently, there has been a proposal (refer to JP2012-7071A) regarding an ink composition in which a thioxanthone photopolymerization initiator containing a salt of a hydroxyl group, a carboxyl group, or a sulfo group and an α-aminoacetophenone photopolymerization initiator containing a salt of a hydroxyl group, a carboxyl group, or a sulfo group are jointly used, but there has been a demand for performance to be further improved.